The present invention relates, in general, to semiconductor processing methods, and more particularly to a novel method of diffusing dopants into a semiconductor wafer.
In the past, solid diffusion sources have been employed by the semiconductor industry as a source of dopants to be diffused into a semiconductor wafer. These prior diffusion sources generally are a disk shaped boron nitride or other ceramic material that contains a desired dopant. The solid diffusion source is typically placed near the semiconductor wafer, then the source and the wafer are heated to diffuse dopants from the source into the wafer.
One disadvantage of this prior method of using solid diffusion sources is thermal conductivity mismatch. Once diffusion is complete, the diffusion source and the wafer are cooled. Due to the thermal conductivity mismatch between the diffusion source and the wafer, the diffusion source cools at a slower rate than the wafer thereby preventing rapid cooling of the wafer. Consequently, dopants within the wafer continue to diffuse until the diffusion source cools. This uncontrolled diffusion generally results in a doping variation of between eight and ten percent across a semiconductor wafer. Such a nonuniform doping concentration is much higher than the desired variation of less than three percent.
In addition, these prior diffusion sources have a hydrophillc characteristic which results in a variable moisture content. Since thermal conductivity is a function of the moisture content, this hydrophilic characteristic further varies the thermal conductivity of these previous diffusion sources thereby further varying dopant concentrations across a semiconductor wafer.
In order to reduce contamination during the diffusion operation, the diffusion is typically performed while an inert gas flows between the diffusion source and the wafer. This gas flow disturbs the migration of dopants from the diffusion source through the gas to the wafer, and adds to the non-uniform doping concentration across the wafer.
Accordingly, it is desirable to diffuse dopants from a diffusion source into a semiconductor wafer by using a solid diffusion source that cools at a rate substantially equal to the semiconductor wafer, by using a solid diffusion source that is not hydrophilic, and by using a gas flow that does not disturb migration of dopants from the source to the wafer.